Controlling circuit for an LED driver and controlling method thereof

ABSTRACT

The present invention relates to a controlling circuit and controlling method for an LED driver implemented as a flyback topology. The controlling circuit may be at a primary side of a transformer of the LED driver, and include a sampling circuit, an on time sensing circuit of an output diode, a regulating signal generator, and a PWM controller. The sampling circuit may generate a sampling signal indicating output current by sampling at the primary transformer side. The on time sensing circuit can detect an on time of the output diode. The regulating signal generator can generate a regulating signal by regulating the sampling signal, a voltage reference, and the on time of the output diode. The PWM controller may generate a controlling signal to control operation of a switching device of the LED driver to maintain a substantially constant output current in accordance with the regulating signal.

RELATED APPLICATIONS

This application is a continuation-in-part of the following application,U.S. patent application Ser. No. 13/329,614, filed on Dec. 19, 2011, nowissued as a U.S. Pat. No. 8,773,047, and which is hereby incorporated byreference as if it set forth in full in this specification, and whichalso claims the benefit of Chinese Patent Application No.201010619845.0, filed on Dec. 30, 2010, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention generally pertains to electronic circuits, andmore particularly to a controlling circuit and method of controllingdriver for a light emitting diode (LED).

BACKGROUND

With rapid development and continuous innovation in the lightingindustry, and the growing importance of energy savings and environmentalprotection, LED lighting rapidly developed as an important lightingtechnology. However, the luminance of LED lighting (associated with theparameter of luminance intensity) is in direct proportion with thecurrent and forward voltage drop of the LED, and is also varied withtemperature. Therefore, a constant current generator may be necessary todrive the LED. Conventional constant current typically use an opticalcoupler, voltage reference, and sensing circuit as part of an outputregulating circuit.

However, such conventional constant current generators have severaldrawbacks. For example, optical coupler may deteriorate over time, andthe transfer ratio of current may decay, resulting in a disadvantageouseffect on the stability and life for some applications. Also, increasedspace may be occupied, resulting in increased costs due to the largenumber of devices, which also may result in relatively low reliability.Further, the sensing circuit usage may result in increased power lossand lower efficiency. Therefore, conventional LED drivers may not meetthe needs of smaller size, higher efficiency, and increased energysavings.

SUMMARY

In view of the above-mentioned limitations, particular embodiments mayprovide a primary sided controlling circuit and controlling method thatemploys a direct sampling signal of primary side of the transformer tosimplify the circuit structure and improve the efficiency.

In one embodiment, a controlling circuit for a light-emitting diode(LED) driver may include a transformer, an output diode arranged at asecondary side of the transformer, and a switching device arranged at aprimary side of the transformer to form a flyback topology, where thecontrolling circuit is arranged at the primary side of the transformer.The controlling circuit can include: (i) a sampling circuit configuredto generate a sampling signal that indicates an output current signal ofthe LED driver at the primary side of the transformer; (ii) an on timesensing circuit configured to sense an on time of the output diode;(iii) a regulating signal generator configured to regulate the samplingsignal, a voltage reference that is directly proportional to an expectedoutput current of the LED driver, and the on time of the output diode,where the sampling signal and the on time of the output diode are indirect proportion with the voltage reference and in inverse proportionwith a switching cycle, and where the regulating signal generator isconfigured to generate a regulating signal; and (iv) a pulse-widthmodulation (PWM) controller configured to generate a controlling signalto control operation of the switching device of the LED driver based onthe regulating signal to maintain a substantially constant outputcurrent of the LED driver.

In one embodiment, a controlling method for an LED driver configured ina flyback topology, can include: (i) sampling current flowing through aprimary side of a transformer of the LED driver by a sensing resistorarranged in series with a switching device at the primary side of theLED driver, and generating a sensing voltage signal in response thereto;(ii) sampling and holding the sensing voltage signal to obtain a peakvalue of the sensing voltage signal; (iii) generating an averagingsignal from the peak value of the sensing voltage signal, a voltagereference, and an on time of output diode of the LED driver; (iv)generating a regulating signal by compensating the averaging signal; and(v) generating a controlling signal based on the regulating signal tocontrol operation of the switching device of the LED driver to maintaina substantially constant output current.

In one embodiment, a controlling method for an LED configured in aflyback topology, can include: (i) sampling current flowing through aprimary side of a transformer of the LED driver by a sensing resistorarranged in series with a switching device at the primary side of theLED driver, and generating a sensing voltage signal in response thereto;(ii) averaging the sensing voltage signal in accordance with an on timeof an output diode and a switching cycle to generate an averagingsignal; (iii) calculating an error between the averaging signal and avoltage reference to generate an error signal; (iv) generating aregulating signal by compensating the error signal; and (v) generating acontrolling signal to control operation of a switching device of the LEDdriver based on the regulating signal to maintain a substantiallyconstant output of the LED driver.

Embodiments of the present invention can advantageously provide severaladvantages over conventional approaches, due to only the primary side ofthe transformer being employed without an optical coupler and outputsensing circuit of the feedback loop. For example, the stability andoperating life may be increased due to the non-participation of anoptical coupler. In addition, less space, lower cost, and higherreliability can be achieved due to using fewer devices, and higherefficiency may be achieved as a result. Other advantages of the presentinvention will become readily apparent from the detailed description ofpreferred embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional LED driver employingsecondary sided controlling scheme.

FIG. 2 is a schematic diagram of a first example controlling circuit ofan LED driver in accordance with embodiments of the present invention.

FIG. 3A is a schematic diagram of a second example controlling circuitof an LED driver in accordance with embodiments of the presentinvention.

FIG. 3B shows operation waveform examples of the controlling circuit ofan LED driver of FIG. 3A.

FIG. 4 is a schematic diagram of the first example regulating signalgenerator of controlling circuit for an LED drive in accordance withembodiments of the present invention.

FIG. 5 is a schematic diagram of the second example regulating signalgenerator of controlling circuit for an LED drive in accordance withembodiments of the present invention.

FIGS. 6, 7, and 8 are flow diagrams of example controlling methods foran LED driver in accordance with embodiments of the present invention.

FIG. 9 is a schematic block diagram of an example PWM controller inaccordance with embodiments of the present invention.

FIG. 10 is a waveform diagram of an example quasi-resonance controlmode, in accordance with embodiments of the present invention.

FIG. 11 is a schematic block diagram of an example constant on timegenerator in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set fourth in order to provide a thoroughunderstanding of the present invention. However, it will be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

Some portions of the detailed descriptions which follow are presented interms of processes, procedures, logic blocks, functional blocks,processing, schematic symbols, and/or other symbolic representations ofoperations on data streams, signals, or waveforms within a computer,processor, controller, device and/or memory. These descriptions andrepresentations are generally used by those skilled in the dataprocessing arts to effectively convey the substance of their work toothers skilled in the art. Usually, though not necessarily, quantitiesbeing manipulated take the form of electrical, magnetic, optical, orquantum signals capable of being stored, transferred, combined,compared, and otherwise manipulated in a computer or data processingsystem. It has proven convenient at times, principally for reasons ofcommon usage, to refer to these signals as bits, waves, waveforms,streams, values, elements, symbols, characters, terms, numbers, or thelike.

Furthermore, in the context of this application, the terms “wire,”“wiring,” “line,” “signal,” “conductor,” and “bus” refer to any knownstructure, construction, arrangement, technique, method and/or processfor physically transferring a signal from one point in a circuit toanother. Also, unless indicated otherwise from the context of its useherein, the terms “known,” “fixed,” “given,” “certain” and“predetermined” generally refer to a value, quantity, parameter,constraint, condition, state, process, procedure, method, practice, orcombination thereof that is, in theory, variable, but is typically setin advance and not varied thereafter when in use.

Embodiments of the present invention can advantageously provide severaladvantages over conventional approaches, due to only the primary side ofthe transformer being employed without an optical coupler and outputsensing circuit of the feedback loop. For example, the stability andoperating life may be increased due to the non-participation of anoptical coupler. In addition, less space, lower cost, and higherreliability can be achieved due to using fewer devices, and higherefficiency may be achieved as a result. The invention, in its variousaspects, will be explained in greater detail below with regard toexemplary embodiments.

Referring now to FIG. 1, a schematic diagram of a conventional off linelight-emitting diode (LED) driver with constant current controllingscheme is depicted. In this example, an isolated flyback converter mayinclude transformer 101, power switching device 102, pulse-widthmodulation (PWM) controller 103, and optical coupler 104, as well as anadditional regulating circuit for the output current.

To maintain the output current of the LED driver to a substantiallyconstant predetermined value under varied conditions, the output voltageof the secondary side of transformer 101 may be detected, and thencompared against a voltage reference to generate an error signal. Theerror signal may then be transferred to the PWM controller 103 at theprimary side of transformer 101 by optical coupler 104, and then used tocontrol the duty cycle of power switching device 102. The output currentcan recover to the initial predetermined value by way of the control foroperation of power switching device 102 implemented by the controllingcircuit at the primary side, in spite of variations of the outputcurrent of LED driver.

However, the optical coupler may deteriorate over time, and the transferratio of current may decay, resulting in a disadvantageous effect on thestability and life for some applications. Also, increased space may beoccupied by the circuit of FIG. 1, resulting in increased costs due tothe relatively large number of devices, which also may result inrelatively low reliability. Further, the sensing circuit usage mayresult in increased power loss and lower efficiency. Therefore, LEDdrivers as shown in the particular example of FIG. 1 may not meet theneeds of smaller size, higher efficiency, and increased energy savings.

Referring now to FIG. 2, an example controlling circuit for an LEDdriver in accordance with embodiments of the present invention is shown.This particular example shows a flyback topology implementation, butother topologies are also accommodated in particular embodiments. Inthis example, an external AC input voltage may be received by anelectromagnetic interference (EMI) circuit and a rectifier bridge and/orpower factor correction (PFC), and then be transferred to the primaryside (Np) of transformer 201.

Switching device 202 may be coupled to the primary winding oftransformer 201, and controlled by controlling circuit 200. Output diode203 and output capacitor 204 can be coupled the secondary winding (Ns)of transformer 201, in order to provide a substantially constant outputcurrent to drive the LED. Controlling circuit 200 can include samplingcircuit 205 arranged at the primary side of transformer 201 to samplethe current information at the primary side. Controlling circuit 200 canalso include on time sensing circuit of output diode 206 at thesecondary side of transformer 201 to sense the on time of output diode203.

Controlling circuit 200 can also include regulating signal generator 207to regulate the received sampling signal Vsample of sampling circuit205, voltage reference Vref, and on time of output diode Tdis.Regulating signal generator 207 may substantially guarantee thatsampling signal Vsample and on time of output diode Tdis are in directproportion with voltage reference Vref, and are in inverse proportionwith switching cycle T, and as a result to generate a regulating signal.This regulating signal may then be supplied to pulse-width modulation(PWM) controller 208.

PWM controlling circuit 208 can generate a controlling signal inaccordance with the received regulating signal to control operation ofswitching device 202 of the LED driver to maintain a substantiallyconstant output current. Here, voltage reference V_(ref) is directlyproportional with expected output current I₀.

In particular embodiments, the controlling may be operated by direct useof a sampling signal at the primary side of transformer 201, withouthaving to sample and output at the secondary side of transformer 201. Inthe LED driver example of FIG. 2, which takes advantages of simplifiedcircuit structure, a relatively low cost and more efficient LED drivercan be produced.

Referring now to FIG. 3A, a controlling circuit for an LED driver inaccordance with embodiments of the present invention is shown as aflyback topology implementation. In this particular example, sensingresistor 305 can connect in series with switching device 202, for use asthe sampling circuit. Sensing voltage Vcs between the two terminals ofsensing resistor 305 May be transferred to regulating signal generator207. Subsidiary winding 306 and differential capacitor 307 can connectin series to form the on time sensing circuit of output diode 206.Subsidiary winding 306 can be used to obtain the current information ofsecondary winding of transformer 201. In addition, on time of outputdiode 203 can be achieved by detecting the voltage Vcd of differentialcapacitor 307.

Referring also to FIG. 3B in conjunction with FIG. 3A, example operationwaveforms associated with the LED driver of FIG. 3A are shown. During anon time T_(on) of switching device 202, the input voltage may besupplied to primary winding Np, and current through the primary windingand switching device 202 can increase linearly from zero to peak valueIppk. Energy of input terminal may then be transferred to primarywinding Np.

During an off time of switching device 202, energy stored in the primarywinding can push output diode 203 to be turned on, and current of outputdiode 203 may decrease from peak value Ispk to zero linearly during theon time of output diode 203, Tdis. During an off time of switchingdevice 202, current through output diode 203 can be represented onsubsidiary winding 306, which can be detected by differential capacitor306 (see, e.g., waveform labeled Vcd).

When the A to B segment of waveform Vcd is detected, signal Vcd can bethe chosen as the rising edge of the on time of output diode Tdis. Whenthe B to C segment of waveform Vcd is detected, signal Vcd can be chosenas the failing edge of the on time of output diode Tdis. In this way, ontime Tdis may be achieved as shown.

In accordance with Ampere's Law, and assuming that the turns of theprimary side of the transformer are np and turns of the secondary sideof the transformer are ns, a relationship between a peak value of outputcurrent Ispk and a peak value of primary side Ippk can be as indicatedbelow in Equation 1.

$\begin{matrix}{I_{spk} = {\frac{n_{p}}{n_{s}} \cdot I_{ppk}}} & (1)\end{matrix}$

Output current I₀ may be equal to and average current of output diode203 during a switching cycle in a constant output current mode, andoutput current I₀ can be calculated as in Equation 2.

$\begin{matrix}{I_{0} = {\frac{1}{2}{I_{Spk} \cdot T_{dis} \cdot \frac{1}{T}}}} & (2)\end{matrix}$

If parameter Ispk is substituted in Equation 1, output current I₀ can becalculated as shown below in Equation 3.

$\begin{matrix}{I_{o} = {\frac{1}{2} \cdot \frac{n_{p}}{n_{s\;}} \cdot I_{ppk} \cdot \frac{T_{dis}}{T}}} & (3)\end{matrix}$

Here, sensing resistor 305 as sensing circuit may be arranged at theprimary side of transformer 201 to detect the peak current value of theprimary side of the transformer, Ippk. If the resistance of sensingresistor is Rs and the peak value of the sensing voltage between the twoterminals of sensing resistor 305 is Vcspk, the peak current of theprimary side of transformer 201 can be calculated as in Equation 4.

$\begin{matrix}{I_{ppk} = \frac{V_{cspk}}{R_{s}}} & (4)\end{matrix}$

In addition, output current I₀ can be determined from Equation 5.

$\begin{matrix}{I_{0} = {\frac{1}{2} \cdot \frac{n_{p}}{n_{s}} \cdot \frac{V_{cspk}}{R_{s}} \cdot \frac{T_{dis}}{T}}} & (5)\end{matrix}$

If voltage signal

$V_{X} = \frac{V_{cspk} \cdot T_{dis}}{T}$can be fixed on the precondition of fixed turns of primary winding np,fixed turns of secondary winding ns, and fixed sensing resistance Rs,output current Io can be substantially constant. Therefore, theregulations of received sampling signal Vcs of sensing resistor 305,voltage reference Vref, and on time of output diode 203 Tdis, of on timesensing circuit 206 can ensure that voltage signal

$\frac{V_{cpsk} \cdot T_{dis}}{T}$maintains a substantially constant value in direct proportion withvoltage reference Vref, and may also generate a regulating signaltherefrom. Further, sampling signal Vcspk and on time of output diodeTdis may be directly proportional with voltage reference Vref, andinversely proportional with switching cycle T.

The value of voltage reference Vref may be predetermined in accordancewith expected output current I₀, resistance Rs of sensing resistor 305,turns of both primary winding np, and secondary winding ns followingEquation 6.

$\begin{matrix}{V_{ref} = \frac{2 \cdot I_{0} \cdot R_{s} \cdot n_{s}}{n_{p}}} & (6)\end{matrix}$

From the above Equations 5 and 6, by the predetermination of voltagereference Vref, and regulation of regulating signal generator 207,voltage signal

$\frac{V_{cpsk} \cdot T_{dis}}{T}$may be in direct proportion with expected output current I₀.

PWM controller 208 can generate a controlling signal in accordance withthe received regulating signal to control operation of switching device202 in order to maintain a substantially constant output current I₀ ofthe LED driver.

In this way, controlling can be operated by direct use of a samplingsignal of the primary side of transformer without sampling of the outputat the secondary, as shown in the LED driver depicted in FIG. 2. Thisapproach takes advantage of a simplified circuit structure, with lowercost, and higher efficiency of the LED driver.

Referring now to FIG. 4, shown is an example of regulating signalgenerator 207 of the LED driver discussed above with reference to FIGS.2 and 3A, and in accordance with embodiments of the present invention.Regulating signal generator 207 can include sampling and holding circuit401 to receive sensing voltage signal Vcs and to obtain the peak valueVcspk of the sensing voltage signal Vcs. Sampling and holding circuit401 can include error amplifier A1, switching transistor M1, andsampling and holding capacitor C1.

As shown in FIG. 3B, at the conversion moment from an on status to anoff status of switching device 202, or the falling edge of the PWMcontrolling signal, a single pulse Psam may be generated to control turnon of switching transistor M1. Then, peak value of sensing voltagesignal Vcspk can be detected to indicate the maximum current Ispk ofoutput diode 203 and a peak value of primary winding of transformer 201.

Current reference generator 402 can include error amplifier A2,switching transistor M2, and resistor R1. A positive terminal of erroramplifier A2 can be coupled to voltage reference Vref, and a negativeterminal of error amplifier A2 can be coupled to a common node ofresistor R1 and switching transistor M2. The other terminal of resistorR1 can be connected to ground, and an output terminal of error amplifierA2 can be connected to switching transistor M2. Current reference Irefmay then be generated through resistor R1, and the value of which can becalculated as in Equation 7.

$\begin{matrix}{I_{ref} = \frac{V_{ref}}{R_{1}}} & (7)\end{matrix}$

Current mirror 406 can be coupled to current reference generator 402 ata first terminal to mirror current reference Iref, and to generatemirroring current at the second terminal.

Sampling current generator 403 can include error amplifier A3, switchingtransistor M3, and resistor R2. A positive terminal of error amplifierA3 can be coupled to sampling and holding circuit 401 to receive signalVcspk, while a negative terminal of error amplifier A3 may be coupled toa common node of resistor R2 and switching transistor M3. The otherterminal of resistor R2 can be connected to ground, and an outputterminal of error amplifier A3 may be coupled to switching transistorM3. Therefore, sensing current Isample can be generated through resistorR2, the value of which can be calculated as below in Equation 8.

$\begin{matrix}{I_{sample} = \frac{V_{cspk}}{R_{2}}} & (8)\end{matrix}$

One terminal of switching transistor 404 may be coupled to an outputterminal of current mirror 406, while the other terminal of which may becoupled to an output terminal of sampling current generator 403. Inoperation, switching transistor 404 can be controlled by an on time ofoutput diode 203, T_(dis).

Compensation circuit 405 can include resistor R3 and capacitor C2connected in series between common node A′ of both switching transistor404 and current mirror 406, and ground to compensate current of commonnode A′.

Therefore, Equation 9 can be derived as shown below.

$\begin{matrix}{{\frac{V_{ref} \cdot T}{R_{1}} - \frac{V_{cspk} \cdot T_{dis}}{R_{2}}} = 0} & (9)\end{matrix}$

That is, as shown below in Equation 10.

$\begin{matrix}{{V_{ref} \cdot \frac{R_{2}}{R_{1}}} = {V_{cspk} \cdot \frac{T_{dis}}{T}}} & (10)\end{matrix}$

Thus, voltage signal

$V_{cspk} \cdot \frac{T_{dis}}{T}$may be directly proportional with voltage reference Vref. Also, voltagereference Vref can be predetermined as indicated below in Equation 11.

$\begin{matrix}{V_{ref} = \frac{2 \cdot I_{0} \cdot R_{s} \cdot n_{s}}{n_{p}}} & (11)\end{matrix}$

Here, parameter I₀ indicates expected output current of the LED driver,Rs indicates resistance of sensing resistor 305, np indicates turns ofprimary winding of transformer, and ns indicates turns of secondarywinding.

With reference to Equations 10 and 11, a voltage signal can be in directproportion with voltage reference Vref through the predetermination ofvoltage reference and regulation of regulating signal generator 207.

PWM controller 208 can be used to generate a controlling signal tocontrol the operation of switching device 202 of the LED driver inaccordance with received regulating signal COMP in order to maintainoutput current I0 as substantially constant.

Referring now to FIG. 5, shown is an example of regulating signalgenerator 207 as in FIGS. 2 and 3A, and in accordance with embodimentsof the present invention. In this example of FIG. 5, regulating signalgenerator 207 can include sampling and holding circuit 501 to sample andhold received sensing voltage Vcs of resistor 305, in order to obtain apeak value of sensing voltage, Vcspk. The peak value of sensing voltage,Vcspk, can be generated by sampling and holding circuit 401 as discussedabove with reference to FIG. 4, or any other suitable type of samplingand holding circuit.

Averaging circuit 502 may average the peak value of sensing voltageVcspk to obtain an averaging signal Vavg. Averaging circuit 502 caninclude switch S1, switch S2, resistor R4, and capacitor C3. Switch S1,resistor R4, and capacitor C3 may be connected in series between theoutput terminal of sampling and holding circuit 501 and ground. Oneterminal of switch S2 can be connected to a common node of switch S1 andresistor R4.

The operation of switch S1 may be controlled by an on time of outputdiode 203 to be substantially consistent with an on status of outputdiode 203. The operation of switch S2 can be controlled by an off timeof output diode 203 to be substantially consistent with an off status ofoutput diode 203. The value of the averaging signal Vavg on the commonnode (B) between resistor R4 and capacitor C3 can be calculated asindicated below in Equation 12.

$\begin{matrix}{V_{avg} = \frac{V_{cspk} \cdot T_{dis}}{T}} & (12)\end{matrix}$

Error amplifier 503 may be used to compare the received averaging signalVavg and the voltage reference Vref. Compensation circuit 504 caninclude capacitor C4 and resistor R5, and may be used to compensate theoutput of error amplifier 503, to output the regulating signal COMP.

The operating principle of error amplifier 503 may support Equation 13as below.V_(ref)=V_(avg)  (13)

If parameter Vref is substituted into Equation 12, voltage referenceVref can be calculated as indicated below in Equation 14.

$\begin{matrix}{V_{ref} = {V_{cspk} \cdot \frac{T_{dis}}{T}}} & (14)\end{matrix}$

As such, voltage signal

$V_{cspk} \cdot \frac{T_{dis}}{T}$can be in direct proportion with voltage reference V_(ref), which may bepredetermined in accordance with expected output current I₀ of the LEDdriver, a resistance of sensing resistor 305, turns of primary windingnp, and turns of secondary winding ns, consistent with Equation 11above.

From the preceding Equations 11 and 14, both the predetermination ofvoltage reference Vref and the regulation of regulating signal generator207 may carry out a direct proportion between voltage signal

$V_{cspk} \cdot \frac{T_{dis}}{T}$and voltage reference V_(ref), and PWM controller 208 may be used togenerate a controlling signal to control operation of switching device202 of the LED driver to maintain a substantially constant outputcurrent in accordance with received regulating signal COMP.

Referring now to FIG. 6, shown is a flow diagram of a first examplecontrolling method for an LED driver in accordance with embodiments ofthe present invention is shown. At S601, sampling can be performed atthe primary side of the transformer of the LED driver to generate asampling signal indicating an output current at the secondary side.

At S602, the received sampling signal, a voltage reference, and an ontime of an output diode at the secondary side of the transformer may beregulated to ensure that the sampling signal and the on time of outputdiode are directly proportional with voltage reference Vref, and ininverse proportion with the switching cycle to generate a regulatingsignal, where the voltage reference may be in direct proportion with anexpected output current of the LED driver.

At S603, a controlling signal may be generated to control the operationof a switching device of the LED driver to maintain a substantiallyconstant output current in accordance with the regulating signal by thePWM controller.

Referring now to FIG. 7, shown is flow diagram of a second examplecontrolling method for an LED driver in accordance with embodiments ofthe present invention. At S701, a sensing resistor connected in serieswith a switching device at the primary side of the LED driver may beused to sample the output current of the primary winding, and togenerate a sensing voltage signal.

At S702 the sensing voltage signal may be sampled and held to generate apeak value of the sensing voltage signal. At S703, an averaging signalmay be generated in accordance with the received peak value of thesensing voltage signal, a voltage reference, and an on time of outputdiode. At S704, the averaging signal may be compensated to generate aregulating signal.

At S705, a controlling signal may be generated to control the operationof a switching device of the LED driver in accordance with theregulating signal by the PWM controller. For example, generation of anaveraging signal can include generating a current voltage in accordancewith the voltage reference, generating a peak value of sampling currentin accordance with the sensing voltage signal, generating the averagingsignal in accordance with the received peak value of sensing current,the current reference, and on time of output diode

Referring now to FIG. 8, shown is a flow diagram of a third examplecontrolling method for an LED driver in accordance with embodiments ofthe present invention. At S801, a sensing resistor connected in serieswith a switching device at the primary side of LED driver may be used tosample the output current of the primary winding, and to generate asensing voltage signal.

At S802, the sensing voltage signal may be averaged to generate anaveraging signal in accordance with the switching cycle and on time ofthe output diode. At S803, the error between the averaging signal and avoltage reference may be calculated to generate an error signal. AtS804, the error signal may be compensated to generate a regulatingsignal.

At S805, a controlling signal may be generated to control the operationof the switching device of the LED driver to maintain a substantiallyconstant output current in accordance with the regulating signal by thePWM controller. The voltage reference may be in direct proportion withan expected output current, a resistance of sensing resistor, and turnsof primary winding, and in inverse proportion with the turns of primarywinding. Also, the averaging signal may be in direct proportion with thesensing voltage and an on time of output diode, and inverselyproportional with the switching cycle.

In this fashion, LED driver controlling can be performed by direct useof a sampling signal of the primary side of the transformer withoutsampling of the output at the secondary side of the transformer. Thus,provided is a controlling circuit and controlling method for an LEDdriver, which takes advantages of substantially constant and stableoutput current, simplified circuit structure, lower cost and improvedefficiency LED driver.

Referring now to FIG. 9, shown is a schematic block diagram of anexample

PWM controller in accordance with embodiments of the present invention.For example, PWM controller 208 as shown in FIG. 2 can be indicated inthe box in FIG. 9. Regulating signal COMP can be input to PWM controller208, and can be the output signal as shown above in FIGS. 4 and 5. InFIG. 9, amplifier 503 and compensation circuit 504 of FIG. 5 are shownto facilitate the description. In this particular example, amplifier 503(e.g., an operational transconductance amplifier) can be configured togenerate an error signal between a feedback signal (e.g., average signalV_(avg) of FIG. 5) and voltage reference V_(ref).

Compensation circuit 504 can be configured to compensate the errorsignal, and to generate regulating signal COMP, and can be any suitableconfiguration circuit (e.g., capacitor, capacitor and resistor coupledin series, capacitor and resistor coupled in parallel, etc.). Oneexample of such compensation circuit 504 is shown in FIG. 5. In PWMcontroller 208, constant on time generator 902 can be configured togenerate a constant on time signal, the duration of which can becontrolled by regulating signal COMP to control, e.g., the off operationof the main power switch (e.g., switching device 202 of FIG. 2). Forexample, on signal generator 904 can be configured to generate an onsignal to control the on operation or state of the main power switch. RSflip-flop 906 can be coupled to constant on time generator 902, and toon signal generator 904, to receive the constant on time signal and theoff control signal, and to generate a PWM control signal. For example,the PWM control signal can control (e.g., via the gate thereof) the mainpower switch (e.g., 202).

For flyback topologies operating in a boundary conduction mode (BCM) ordiscontinuous current mode (DCM) mode, the inductor current can bepresented as below in Equation 15.

$\begin{matrix}{i_{p\; k} = {\frac{V_{in}}{L} \times t_{on}}} & (15)\end{matrix}$

Here, i_(pk) can represent a peak value of the inductor current flowingthrough the primary winding of the transformer (e.g., transformer 201),V_(in) can represent the input voltage, t_(on) can represent the on timeof the main power switch (e.g., switching device 202 of FIG. 2), and Lcan represent the inductance of the primary winding of the transformer.When on time t_(on) is constant, because L is also constant, the peakvalue of inductor current i_(pk) can be in direct proportion with inputvoltage V_(in). As shown in the example of operation waveforms of FIG.10, the envelope line of the peak value can be depicted as a sinusoidalwave. Therefore, the waveform of the input current in_(IN) may also berepresented as a sinusoidal wave that follows input voltage V_(in). Inthis way, power factor correction can be achieved to obtain a high powerfactor for the implementation as shown in FIG. 2, as compared toconventional approaches.

In the example of FIG. 9, in order to maintain on time t_(on) constant,operational amplifier 503 may have a relatively wide linear range (e.g.,from about −200% to about +250%, such as from about −100% to about+150%). When the output current is close to (e.g., within apredetermined amount) or is otherwise consistent with the expectedoutput current, regulating signal COMP may be maintained assubstantially constant, and on time t_(on) can be substantiallyconstant. When the output current is relatively far away from theexpected output current, regulating signal COMP may begin to regulatethe on time until the output current recovers to the expected outputcurrent, and then on time t_(on) can again be maintained assubstantially constant. Through this regulation, both power factorcorrection and constant output current regulation can be achieved inparticular embodiments.

Furthermore, on signal generator 904 can be configured in aquasi-resonance control mode or valley current control mode, or anyother suitable operating modes to control the on operation of the mainpower switch. As shown in FIG. 10, when the voltage between the firstpower terminal and the second power terminal of the main power switch(e.g., drain-to-source voltage V_(DS) when the main power switchconfigured as an N-type MOSFET transistor) reaches a level of a valleyor low/minimum value, the PWM control signal can go high to turn on themain power switch. Any suitable circuit implementation or configurationcan be utilized to achieve quasi-resonance control.

Referring now to FIG. 11, shown is a schematic block diagram of anexample constant on time generator in accordance with embodiments of thepresent invention. This particular example constant on time generatorcan include voltage-controlled current source I_(source) and capacitorC_(charge) coupled in series, switch S_(W) coupled in parallel withcapacitor C_(charge), and comparator 1102 that receives comparisonvoltage reference V_(ref1) and a signal at a common node of voltagecontrolled current source I_(source) and capacitor C_(charge). Forexample, a current of current source I_(source) can be controlled byregulating signal COMP. Also, operation of switch S_(W) can thus beopposite to that of the main power switch, and switch S_(W) can becontrolled by inverting the PWM control signal.

When the main power switch is turned on via the PWM control signal (andswitch S_(W) is off), capacitor C_(charge) can be charged by currentsource I_(source). The voltage across capacitor C_(charge) can increaselinearly and continuously, until the voltage across capacitor C_(charge)reaches a level of voltage reference V_(ref1). As a result, the PWMcontrol signal can go low to turn of the main power switch (e.g.,switching device 202). Correspondingly, switch S_(W) can be on andcapacitor C_(charge) can be discharged to ground through the turned-onswitch S_(W). Also, the output signal of comparator 1102 can beconfigured as the on time control signal.

In particular embodiments, because amplifier 503 may have asubstantially wide linear range, the output signal of amplifier 503 canbe utilized to achieve both constant on time and constant currentregulation. In a certain range, regulating signal COMP can be constant,and the on time can be constant as a result. Also, when the outputcurrent is relatively far away from an expected output current, the ontime can be regulated to correct and bring the output current back inline with the expected output current. In addition, a constant on timecontrolling mode can be utilized to achieve both power factor correctionand constant output current regulation.

The foregoing descriptions of specific embodiments of the presentinvention have been presented through images and text for purpose ofillustration and description of the LED driver controller circuit andmethod. They are not intended to be exhaustive or to limit the inventionto the precise forms disclosed, and obviously many modifications andvariations are possible in light of the above teaching, such as thealternatives of the type of switching device, on time sensing circuit ofoutput diode, controlling of switching device and sampling and holdingcircuit for different applications.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A controlling circuit for a light-emitting diode(LED) driver having a transformer, said controlling circuit comprising:a) a sampling circuit configured to generate a sampling signal thatindicates an output current signal of said LED driver at a primary sideof said transformer; b) an on time sensing circuit configured to sensean on time of an output diode arranged at a secondary side of saidtransformer; c) a regulating signal generator configured to generate aregulating signal based on said sampling signal, a voltage referencethat is directly proportional to an expected output current of said LEDdriver, and said on time of said output diode, wherein said regulatingsignal generator comprises an operational amplifier; and d) apulse-width modulation (PWM) controller configured to generate acontrolling signal to control operation of a switching device of saidLED driver based on said regulating signal, and wherein an on time ofsaid switching device is controlled by said regulating signal.
 2. Thecontrolling circuit of claim 1, wherein said operational amplifiercomprises a linear range from about −200% to about +250%.
 3. Thecontrolling circuit of claim 1, wherein aid operational amplifiercomprises a linear range from about −100% to about +150%.
 4. Thecontrolling circuit of claim 1, wherein said on time of said switchingdevice is controlled to be constant by said regulating signal to achievepower factor correction.
 5. The controlling circuit of claim 1, whereinsaid sampling circuit comprises: a) a sensing resistor coupled in serieswith said switching device at said primary side of said transformer,configured to convert current flowing through said primary side of saidtransformer to a sensing voltage signal; and b) a sampling and holdingcircuit configured to generate a peak value of said sensing voltagesignal.
 6. The controlling circuit of claim 1, wherein said regulatingsignal generator comprises: a) said operational amplifier configured togenerate an output signal that represents an error between said voltagereference and an averaging signal, wherein said output signal isconfigured for constant on time and constant current regulation; and b)a compensation circuit configured to generate said regulating signalbased on said output signal of said operational amplifier.
 7. Thecontrolling circuit of claim 6, wherein said averaging signal is indirect proportion with said on time of said output diode and said peakvalue of sensing voltage signal, and in inverse proportion with aswitching cycle of said switching device.
 8. The controlling circuit ofclaim 6, wherein said averaging circuit comprises: a) a first switchhaving a first terminal coupled to receive said peak value of saidsensing voltage signal, and a second terminal coupled to a firstterminal of a resistor and a first terminal of a second switch, whereina second terminal of said second switch is coupled to ground; and b) acapacitor having a first terminal coupled to a second terminal of saidresistor and an average node, and a second terminal coupled to ground,c) wherein said first switch is configured to be turned on when saidoutput diode is on, and said second switch is configured to be turnedoff when said output diode is off.
 9. The controlling circuit of claim1, wherein said regulating signal generator comprises an averagingcircuit configured to average said peak value of said sensing voltagesignal in accordance with said on time of said output diode, and togenerate an averaging signal.
 10. The controlling circuit of claim 1,wherein when a product of said sampling signal and said on time of saidoutput diode is in direct proportion with a product of said voltagereference and a switching cycle of said switching device, an outputcurrent of said LED driver is consistent with said expected outputcurrent.
 11. The controlling circuit of claim 1, wherein said voltagereference is directly proportional to a ratio of a product of saidexpected output current and turns of a secondary winding of saidtransformer, and turns of a primary winding of said transformer.
 12. Thecontrolling circuit of claim 1, wherein said PWM controller comprises:a) a constant on time generator configured to generate an on time signalin accordance with said regulating signal; b) an on signal generatorconfigured to generate an on control signal to turn on said switchingdevice; and c) an RS flip-flop configured to be set by an output fromsaid on signal generator, and to be reset by an output from saidconstant on time generator, wherein an output of said RS flip-flop isconfigured as said controlling signal.
 13. The controlling circuit ofclaim 12, wherein said on signal generator is configured to achieve aquasi-resonant control mode.
 14. The controlling circuit of claim 13,wherein when a voltage between a first power terminal and a second powerterminal of said switching device reaches a level of a valley value,said switching device is configured to be turned on.
 15. The controllingcircuit of claim 12, wherein said constant on time generator comprises:a) a current source having a current controlled by said regulatingsignal; b) a capacitor coupled in series with said current source; c) anon control switch coupled in parallel with said capacitor, wherein an onand off state of said on control switch is inverted with said switchingdevice; and d) a comparator configured to receive a comparison voltagereference and a voltage across said capacitor is configured to generatesaid on time signal.
 16. A method of controlling a light-emitting diode(LED) driver configured in a flyback topology, the method comprising: a)generating a sensing voltage signal by sampling current flowing througha primary side of a transformer of said LED driver; b) determining apeak value of said sensing voltage signal by sampling and holding saidsensing voltage signal; c) generating, by an operational amplifier, aregulating signal based on said peak value of said sensing voltagesignal, a voltage reference, and an on time of an output diode of saidLED driver; and d) generating a controlling signal based on saidregulating signal to control operation of a switching device of said LEDdriver, wherein an on time of said switching device is controlled bysaid regulating signal.
 17. The method of claim 16, wherein saidgenerating said regulating signal comprises: a) generating an averagingsignal by averaging a peak value of said sensing voltage signal inaccordance with an on time of an output diode and a switching cycle; b)generating an error signal representing an error between said averagingsignal and said voltage reference; and c) generating a regulating signalby compensating said error signal.
 18. The method of claim 17, whereinsaid averaging signal is in direct proportion with said on time of saidoutput diode and said peak value of said sensing voltage signal, and ininverse proportion with said switching cycle.
 19. The method of claim16, wherein when a product of a sampling signal and said on time of saidoutput diode is in direct proportion with a product of said voltagereference and a switching cycle of said switching device, and an outputcurrent of said LED driver is consistent with said expected outputcurrent.
 20. The method of claim 16, wherein: a) said voltage referenceis directly proportional to a ratio of a product of said output currentand turns of a secondary winding of said transformer, and turns of aprimary winding of said transformer; b) said on time of said switchingdevice is controlled to be constant by said regulating signal to achievepower factor correction; and c) said switching device is controlled tooperate in a quasi-resonant control mode.